Almost all wireless body sensor network applications require synchronization of data originating from different sensor nodes. Time synchronization protocols such as the Flooding Time Synchronization Protocol (FTSP) provide a solution for synchronizing all nodes belonging to the same network. FTSP relies on a time master that periodically broadcasts time synchronization messages which are time-stamped at the MAC layer. The receiving nodes compare their local arrival time of the time synchronization message with the time stamp contained in the time synchronization message and use it for estimating the clock skew relative to the time master to synchronize their clocks.
Low-power low-rate radio technologies such as IEEE 802.15.4/ZigBee have been designed for wireless sensor networking and also the first choice for body sensor network applications. However, the aggregated raw data rate of IEEE 802.15.4/ZigBee is limited to 250 kbit/s per channel, which translates to a usable bandwidth for applications of around 100 Kbit/s or 13 Kbytes/s. Thus, applications requiring more bandwidth (such as body motion capturing systems, EEG, EMG) generally partition the system into multiple separate networks operating on different radio channels.
This raises the problem of time synchronization across networks operating on different channels. Applying the FTSP protocol in a straight forward manner would require to have one time master per network and to synchronize all time masters with each other, which leads to a complex and complicated solution.